feat(settings): Settings page with email, webhooks, notifications, profile [FASE-8]

- SystemConfig model + migration b033 for runtime key-value config
- GET/PATCH /system/email-config + POST /system/email-test (admin only)
- email_service reads SMTP config from DB (overrides .env)
- Webhooks now accessible to red_lead/blue_lead + admin
- GET /users/me already existed; /users/me/preferences already working
- SettingsPage with 4 role-aware tabs:
  * Profile & Jira: jira_account_id, user info
  * Notifications: role-specific email/in-app toggles (12 prefs)
  * Webhooks: full CRUD + test ping (leads + admin)
  * Email/SMTP: enable toggle, server config, test email (admin only)
- Added /settings route (all authenticated users)
- Settings link added to Sidebar

backend/app/models/system_config.py

@@ -0,0 +1,26 @@
+"""SystemConfig model — runtime key-value configuration store."""
+
+import uuid
+
+from sqlalchemy import Column, String, Text, DateTime, func
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.database import Base
+
+
+class SystemConfig(Base):
+    """Generic key-value store for runtime system configuration.
+
+    Currently used for:
+    - SMTP email settings (overrides .env values when present)
+
+    Keys are namespaced by convention: ``smtp.host``, ``smtp.port``, etc.
+    """
+
+    __tablename__ = "system_configs"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    key = Column(String(200), unique=True, nullable=False, index=True)
+    value = Column(Text, nullable=True)
+    description = Column(String(500), nullable=True)
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())